It is not more difficult to radar-track an object (such as the helicopter sent out to look for Captain and his gunman), if they fly in zig-zag. No matter what direction or heading, the radar-tracking would be the same.
When Chandler and Tex are evading the Russian boats Chandler orders Tex to "Go to the .50 cal...". The weapon mounted on the RHIB is an M240B which is chambered in 7.62mm NATO and could not remotely be mistaken for a .50 M2 Browning. An error an experienced Commander would not likely make.
It's possible that an immune person could propagate an illness. For a doctor, to be without any protection in the same room with a person that had been in contact with a very contagious and mortal illness, and that hadn't been quarantined, would be a gross negligence.
The girl rescued from the boat, would not have been let on board, or even approached by anyone. Prior to being tested for carrying the virus.
Even though she might be immune, this does not mean she is not a carrier. Even extremely likely to be a carrier since she has spent so much time close to it.
The Russians would have the Nathan James vessel located in a matter of seconds with all the radio-communications going on between the Captain's crew, that is trying to rescue the girl, and their Destroyer.
When the RHIBs are racing towards the stranded vessel Octopus to rescue Bertrise, we see the boats bouncing over waves throwing up wake and there on the gunwale is a US flag hanging limply, with no breeze at all.